Method for the preparation and use of a catalyst for the production of polyesters, more particularly high molecular weight linear polyesters and catalyst thus obtained

ABSTRACT

Method for the preparation of finely and evenly divided elemental antimony for use as catalyst in polyester manufacturing, wherein powdered antimony is subjected to mechanical size reduction operations, and then is dispersed in a liquid medium.

United States Patent 72 Inventors Francesco Slclarl Cesano Maderno;Giuseppe Messina, Llmbiate; Edgardo Honk, Bnrlassina, all of Italy [2|Appl. No. 784,248

[22] Filed Dec. I6, 1968 [45] Patented Nov. 30, I971 [73 Assignee SulaViscose Societal Nazlouale Industria Appllcazlonl Viscose S.p.A. Milan,Italy [32] Priority Dec. 23, 1967 133 Italy [54] METHOD FOR THEPREPARATION AND USE OF A CATALYST FOR THE PRODUCTION OF POLYESTERS, MOREPARTICULARLY IIIGII MOLECULAR WEIGHT LINEAR POLYESTERS AND CATALYST THUSOBTAINED 8 Claims, No Drawings [52] U.S. Cl. 260/75, 252/46l, 252/430 [51] Int. Cl C08g l7/00 [50] Field ol Search 252/461, 430;260/75;24l/l6[56] References Cited UNITED STATES PATENTS 2,274,766 3/1942 Zirhc106/290 2,065,762 l2/l936 Stanley 83/94 FOREIGN PATENTS 740,38 l 2/1953England 260/75 OTHER REFERENCES Goetzec, Treatise on Powder MetallurgyVol. I p. 201 Interscience Pub. Inc. New Y ork 1949 PrimaryExaminer-Daniel E. Wyman Assistant E.raminer-Philip M. French Auomey- B.Edward Shlesinger ABSTRACT: Method for the preparation of finely andevenly divided elemental antimony for use as catalyst in polyestermanufacturing. wherein powdered antimony is subjected to mechanical sizereduction operations. and then is dispersed in a liquid medium.

METHOD FOR THE PREPARATION AND USE OF A CATALYST FOR THE PRODUCTION OFPOLYESTERS, MORE PARTICULARLY HIGH MOLECULAR WEIGHT LINEAR POLYESTERSAND CATALYST THUS OBTAINED This invention relates to a method forpreparing finely and evenly divided elemental antimonium which has thephysical and general characteristics required for a catalyst to be usedin the preparation of linear polyesters.

It is known that, in the preparation of polyesters, and moreparticularly linear polyesters adapted to be converted into fibers,films, tapes and the like, it is common practice to react a dialkylester (in which the alkyl has a low molecular weight) of a bicarboxylicorganic acid, or, as an alternative, the bicarboxylic acid itself, witha diol, and more particularly with a diol of the series HO(CH ),,OHwherein n is an integer comprised between two and 10.

As a general rule, the starting materials are dimethylterephthalate, orthe terephthalic acid as such, and, among the diols, ethylene glycol isthe most commonly adopted one.

When starting from a dialkyl ester and a diol, the first stage of themethod is an ester interchange of the two reactants, which is generallycalled a reesterification reaction. In the most common instance in whichdimethylterephthalate and ethylene glycol are used, this first stagegives bis-(2-hydroxyethyl) terephthalate, which is the monomer to bepolymerized in the subsequent stage in order to convert it into the endproduct, that is, the linear polyester. Said reesterification reactionis usually carried out in the presence of a catalyst in order that thereaction in question be accelerated, and the catalysts which are mostused to this purpose are, for example, the organic salts of zinc,manganese or salts of other metals.

The product obtained from said ester interchange is subsequentlypolymerized or polycondensed until a linear polyester having a highmolecular weight is obtained, which is adapted to be converted intoimportant commercial products, more particularly fibers or yarns.

It is known that also the polymerization or polycondensation reaction iscarried out, as a rule, in the presence of a suitable catalyst. The useof antimony, in different forms and sizes, as a dust, filings and thelike, has already been disclosed, among several metals, as a catalystfor carrying out both the initial stage which is conducive to themonomer, and, still more particularly, the second stage, that is to say,the polycondensation.

Also in the case in which the bicarboxylic acid and the diol are thestarting materials, more particularly terephthalic acid and ethyleneglycol, the monomer is obtained in the first stage and should then bepolycondensed to originate the desired end polymer, and it is knownthat, in both these stages, catalysts are conventionally used toaccelerate the process run, such as compounds of bivalent or trivalentmetals, for example salts of calcium, zinc and others.

Obviously, the use of powdered antimony gives variable results, whichare more or less satisfactory, and are a function, inter alia, of thefineness of the component particles. According to principles andconceptions which are well known to the chemists, a granular catalystmust have such a size as to become finely dispersed within the reactionmass and more particularly, in the present case, in the mass of themonomer, for example bis-(2hydroxyethyl) terephthalate to be condensed.

As a matter of fact, the presence of antimony particles having toocoarse a size, for example 80 to I microns, would not allow the metaldispersed in the reaction mass to offer a contact surface with the masswhich is high enough for the catalyst action to take place in a mostcomplete and satisfactory manner as far as practicable.

It has now been ascertained that metallic antimony, as obtained with theconventional methods, even in particles having a size less than the oneindicated above, for example comprised between a few microns and a fewl0s of microns, has the shortcoming of consisting of particles of unevensize which, within the reaction mass, do not produce a uniformdispersion of the metal, the sizes of said particles varying at randomwithin a considerably wide range.

- It has been ascertained that the use of a catalyst formed by saiduneven particles, is conducive to a unsatisfactory polymers, orfilaments or yams, due to the presence therein of colored or darkparticles and/or lumps. It is possible that this phenomenon is due tothe fact that, during the treatments, the particles may have becomeaggregated or clustered to form units having a size greater than theoriginal one, these units then remaining undissolved in the polymer andin the yarns produced thereby.

Several methods are known in the art for preparing elemental antimony asa catalyst for the production of polymers. The conventional methods areconcerned both with catalysts having a size between a few microns and afew 10's of microns or greater, as outlined above, and catalysts havinga smaller size, whose use is likewise known in the polymer productionart.

In general, these methods require chemical and physical means whichhardly ensure a satisfactory reproducibility of the results. lnaddition, these methods could be conducive to catalysts incapable ofaffording to the polymer the desirable whiteness rating. Among theseveral known methods, the most commonly used are the chemical reductionof antimony com pounds and the grinding of powders, while moreintriguing and difficult methods are available, such as the electricreduction to powder of a metal dispersed within a liquid, the thermaldecomposition of antimony compounds, the grinding of elemental antimonywith the aid of airjets and others.

Powdered elemental antimony of commercial grade is formed by particleshaving a size which is generally over 10 microns, whose average value isbetween 10 and 1000 microns. The elemental antimony which can beobtained by reduction chemical reactions is formed by particles whosesize varies according to the method and the conditions under which thereaction has been carried out, said particles consisting of aggregatesof unit particles, having a rather coarse size, for example over microns(for example 250 microns), or unit particles having a lesser size, forexample lO-lOO microns.

ln any case, commercial antimony, or antimony which is obtained withchemical reduction methods does not give acceptable results if used as acatalyst in said polycondensation reaction, inasmuch as it is formed byparticles, or particle aggregates, having too large a size or having anirregular size distribution.

According to the invention, a simple and quick method has been devised,which is based on physical treatments only, for the preparation ofelemental antimony in fine particles, the latter exhibiting apredetermined and constant uniformity of grit size and being adapted tobe used as a highly efficient catalyst in the production of polyesters,more particularly high-molecular-weight linear polyesters.

Said physical treatments can be applied to elemental antimony in powderform as obtained from the market, or from conventional chemicalreactions, since the elemental or the antimony resulting fromconventional reactions is unacceptable as a catalyst in apolycondensation reaction.

The inventive method is characterized in that powdered elementalantimony, formed by particles or particle-aggregates havingsubstantially a diameter over l0 microns, is subjected to physicalsize-reducing treatments, homogenization and dispersion, such as topermit the obtention of an antimony dispersion formed by particleshaving substantially a size in the order of 10-20 microns, in a liquidwhich is compatible with the subsequent use as a catalyst, the elementalantimony thus obtained being a catalyst which has a high efficiency forthe production of polyesters, more particularly linear polyesters havinga high molecular weight.

According to the invention, the powdered elemental antimony is dispersedwithin said liquid, usually ethylene glycol,

in the presence of an inert gas to prevent oxidation, preferably withthe aid of a mechanical stirrer rotated at a high speed (preliminarydispersion) and the subjected to one or more treatments in a colloidmill, still in the presence of an inert gas, said treatment giving as aresult a dispersion of antimony particles having a size in the order ofl-20 microns. Said final dispersion is adapted to be directly used as acatalyst in the production of polyesters. It can be introduced in theinitial reesterification stage, or, as an alternative, it can be addedto the reaction mass at the outset of the polycondensation stageaforementioned.

in a similar manner, when the process involves direct esterification ofthe bicarboxylic acid and the diol, as outlined above, and thesubsequent polycondensation stage, said dispersion can be added to thereaction mass during the first stage or at the beginning of the secondstage. As a matter of fact, the elemental antimony obtained according tothe invention can be used as a catalyst in both the esterification andpolycondensation stages; it exhibits, however, its best propertiesduring progress of the polycondensation stage.

lt is consequently preferable that the antimony prepared according tothe invention be associated, in said first stage, with a conventionalesterification catalyst, such as a zinc salt, a calcium salt and others.

Preferably, said preliminary dispersion in the liquid by a mechanicalstirrer rotated at a high speed is effected on dispersions having aconcentration of antimony of from 0.1 to 1 percent.

The final stage in the preparation of the catalyst, that is thetreatment in the colloid mill, is preferably carried out as a singlestep at room temperature, although different temperature might also beused.

Subsequent to said final stage, a settling treatment could be carriedout, during a time of from a few minutes, for example minutes, to 6hours, followed by the drawing off of the supernatant clear layer whichcontains the finest particles.

The utility of carrying out said settling step is a function of themanner in which the previous treatments have been performed: it can besuccessfully used whenever the previous treatments have been conductedin such a way as to produce a large number of particles having adiameter which is in the neighborhood of microns.

The settling step can be dispensed with, conversely, when the dispersionobtained has particles whose size distribution is satisfactorily uniformand between 10 and 20 microns, which occurs in the majority ofinstances.

The dispersion of antimony particles obtained according to the inventivemethod contains particles having an average size within a very narrowrange of values, and, more exactly, from ID to 20 microns.

The size of the particles is thus very uniform and the dispersion has agrey color and can be easily admixed with and incorporated in thereaction mass in the process for producing polyesters, and without lumpformation.

For checking purposes, the dispersion obtained after the treatment inthe colloid mill, is subjected to at least two consecutive filtrationsthrough porous diaphragms, the first of which has pores whose nominalsize is from 15 to 40 microns, and the second one has pores with anominal size of from 5 to 15 microns.

lt has been ascertained that said dispersion passes through the firstporous diaphragm entirely, without leaving any appreciable residue onthe diaphragm. The dispersion thus filtered, passed through the secondporous diaphragm, leaves on the diaphragm a grey residue, which isclearly visible, whereas the filtrate does not show any appreciableamount of antimony particles.

The controls made by passing the dispersions through the two porousdiaphragms are a confirmation of the results obtained on samples of saiddispersion with microscopical examinations carried out according to theconventional techniques.

When the starting material for performing the method according to theinvention consists of powdered antimony, whose particles have, as anaverage, a size considerably greater than microns, as frequently occurswith commercial grade antimony, it is wiser to carry out two preliminarydry-grinding and wet grinding treatments on said material, so as toreduce at least the major portion of said particles to a size of lessthan 20 microns, then effecting said preliminary dispersion in a liquidand the subsequent treatment in the colloid mill.

The two preliminary treatments aforementioned are intended to reduce thecomparatively great size of said particles to such a value that they canbe readily processed by said preliminary dispersion and the colloidmill, to obtain the desired size.

In the case in which the starting material consists of powderedantimony, composed of aggregates of particles having a size in excess of100 microns, or of particles with a diameter between 10 and 100 microns,as is the case when antimony is the result of chemical reductionoperations, then said preliminary treatments are redundant, inasmuch asthe particles can be reduced to the desired size without any appreciabledifficulty.

The first of said preliminary treatments consists in one or moredry-grinding operations to give particles having an average diameter ofless than 35 microns. These dry-grinding operations are preferablycarried out by treating the powdered antimony in a ball mill rotated athigh speed, and causing the thusly ground product to pass throughscreens having 16,000- mesh gauzes, so as to permit that particleshaving a diameter substantially below 35 micron be detected.

The resulting product is then subjected to the second preliminarytreatment which comprises one or more grinding operations within aliquid, preferably ethylene glycol, to give particles having an averagediameter of less than 20 microns.

Preferably, the wet-grinding operation is carried out by dispersing theantimony powder as obtained from the first treatment, in ethylene glycolat a concentration of l0-50 percent by weight of antimony in the liquid,and grinding said dispersion with ball mills or sandmills.

Said dimensions of the particles can be determined with the conventionalanalytical tests, for example, microscopical examinations of therelevant samples.

The product obtained is properly dispersed (preliminary dispersion) andthen subjected in the colloid mill to a dispersion and homogenizationaction which leads to the desired end product according to theinvention.

It has been ascertained that the dispersion of antimony, as obtainedaccording to the invention, has the necessary and most desirableproperties for its use as a catalyst in the production of polyesters,and more particularly of linear polyesters.

In order that said properties may be ascertained, polymerization testshave been carried out, by adding the dispersion obtained according tothis invention, to the reaction mass, both in reesterification andpolycondensation processes and in those processes which are based on thedirect esterification of terephthalic acid with a glycol.

Said tests, a few of which are disclosed in the practical examples to bedescribed later, show that the polymerization reaction has asatisfactory short duration and that it takes place in a uniform mannerthroughout the whole reaction mass, that both said mass and the endpolymer have a clear and uniform color, free of lumps and darkparticles. No staining is noted in the chips obtained by extruding thepolymer, and the filaments and yarns produced thereby are devoid ofundesirable discolorations and have the best whiteness characteristics,even retaining, to a high degree, the known textile and mechanicalproperties of said yafns, for example a high tensile strength.

The polymer thus obtained also shows the satisfactory characteristic ofthermal stability; it withstands temperatures of 280-290 C., during longperiods of time without exhibiting any degradation or other alterationsliable to impair the polymer itself.

EXAMPLE 1 A dispersion of elemental antimony in monoethylene glycol inthe form of a fine and uniform particles having a size between and 20microns is prepared according to the following procedure:

a. synthesis of elemental Sb by reacting TiCl with SbCl in an aqueousacidic solution.

Filtration, washing until acidity has been removed with secondaryreaction products, drying in a vacuo until powdered Sb is obtainedhaving a purity of 99 percent or higher, and whose particles or particleaggregates have a size substantially over 10 microns.

b. Preliminary dispersion of powdered Sb in ethylene glycol in theproportion of one part of Sb per 1,000 parts of glycol, by stirring withan Ultraturrax turbine-dispersing machine rotated at 10,000 r.p.m. for 5minutes in an inert environment at room temperature.

c. Treatment of the preliminary dispersion in a colloid mill of theManton-Caulin type. A fraction of the final dispersion obtained ispassed for checking purposes, through porous diaphragms of the Gooch G/3 Jena type having pores with a nominal diameter between and 40microns, without leaving any appreciable residues on the diaphragms. Thedispersion is then passed through porous diaphragms of the G4 Jena type,having pores with a nominal size of 5-15 microns and leaves thereon agrey residue which is clearly visible, whereas no particle passes, invisible manner, through said diaphragms.

Another fraction of the final dispersion thus obtained, looked atthrough a microscope, shows particles having a size substantially offrom 10 and microns.

A portion of said dispersion is used to obtain a spinnable polyethyleneterephthalate polymer, as follows:

A test-polymerization reactor having a capacity of liters and equippedwith a stirrer and distillation columns is charged with the followingmaterials:

dimethyl terephthalate (DMT) 8,000 parts monoethylene glycol (GE) 2,800parts bihydrated zinc acetate 1.6 parts trielhylsulfate 1.33 partsdispersion of Sb (0. 1% in GE) containing 2.4 parts Sb,

equivalent to 0.031 by weight on DMT 2,400 parts The reaction mass isgradually brought to 220 C., during 7 hours, and, during this time allthe methanol formed in the ester interchange reaction of dimethylterephthalate and ethylene glycol is distilled off virtually entirelyfrom the reactor.

A gradual vacuum is then applied to the reaction system so as to attain,during 2 hours, a value of 0.5 to 1 mms. of mercury, the temperaturebeing raised from 220 C., to 280 C.

Polycondensation is completed by maintaining the reaction mass under avacuum of 0.5 mms. of mercury at 280 C., during 3 hours. The formedpolymer is then extruded under nitrogen pressure in cold water.

Said polymer, converted into granular form, is colorless and is watertransparent.

It has, moreover, the following specifications:

[n]=0.673 (intrinsic viscosity, measured at 20 C., in a mixture ofphenol and tetrachloroethane in the weight ratio of 60 to 40, theconcentration of the solution being 0.58 gr. of polymer in I00 mls.

m.p. 265 C.

melt index 0.60 g./l0 mins. as measured at 270 C. with a spinnerethaving a diameter of 0.5 mms. and under a load of2 160 gr.

Number of equivalent C0011 32 (referred to 10 gr. of polymer) Filamentsor yarns obtained from said granules have a clear color and have nolumps or dark spots.

EXAMPLE 2 A sample of elemental antimony, in the form of an impalpablepowder, is obtained by reduction of an aqueous solution of SbC 1,,acidified with hydrogen chloride, with zinc powder. The antimonyparticles, or particle-aggregates, have a size substantially over 10microns.

The powdered Sb is then converted into a 0.1 percent dispersion inethylene glycol according to the procedure indicated under b) of example1 and is then subjected to a pass in a colloid mill of the Manton-caulintype.

Also this dispersion, viewed through a microscope, is essentially formedby particles having a size between 10 and 20 microns.

An amount of 2,400 parts of said dispersion, containing 2.4 parts of Sb,is subjected to a polycondensation test, by using an implementation anda charge according to the same operative conditions as in example 1.

The polymer obtained upon crystallization and drying up to a moisturecontents of less than 0.01 percent has the following specifications [1=0.668 (measured as in example 1 m.p. 265 C.

Melt index 0.61 gr./ 10mins.

No. of CO0H per 10 grs. of polymer 37 Color white with a slight greyhue.

The filaments or yarns obtained by melt-spinning of the granules exhibita satisfactory clear color and do not show any staining or otherundesirable discoloration.

EXAMPLE 3 A specially provided stainless-steel esterification reactorequipped with a stirrer and a rectification column, containing 200 partsof prepolymerized polyethylene terephthalate having a average degree ofpolymerization of 2 kept at a temperature of 245 C., is charge during 1hour with a slurry consisting of:

parts of terephthalic acid 50 parts of monoethylene glycol 0.05 parts ofSb slurried in 25 parts ofGE, obtained as in ex. 2.

The Esterification reaction is carried out under a pressure of 3.5kgs./sq.cm., at a temperature of 245 C., the water formed during thereaction being continually withdrawn for 1 hour. The balance of thereaction water is removed from the reactor by venting the system from3.5 kgs./sq. cm. down to atmospheric pressure, during an additionalhour. The ester thus obtained is transferred to a polycondensationreaction wherein the temperature of the reaction mass is brought, during2 hours, to 280 C. while gradually applying a vacuum, from 760 mms. ofmercury to 1 mm. of mercury.

The polycondensation reaction is carried out within 5 hours at 280 C.under vacuum of l mm./Hg.

The polymer thus obtained is extruded under nitrogen pressure in waterand converted into granular form. The product has the followingspecifications m.p. 263 C.

N of CO0H per 10 gr. of polymer: 3]

Color virtually colorless, water-transparent. Upon crystallization at C.during 30 mins. under nitrogen, the polymer granules exhibit anexcellent whiteness rating.

EXAMPLE 4 A sample of commercial powdered antimony having a grit size of60 mesh (mesh width 250 microns) and a purity of 99 percent, is groundin a rod mill rotated at 15,000 rpm. then screened through a screenhaving a 16,000 mesh gauze until a powder is obtained having a grit sizeof less than 35 microns.

The fine powder thus obtained is admixed with ethylene glycol, up to apercentage of 33.3percent of the whole mixture, until obtaining auniform slurry which is subjected to grinding in a ball mill during 100hours under an inert gas blanket. The fine dispersion thus obtained,which contains particles having a diameter substantially less than 20microns, is diluted to the concentration of 0.1 percent in ethyleneglycol by stirring in a turbine dispersing machine for 5 minutes underan inert gas blanket at room temperature, then it is made homogeneous bytreatment in a colloid mill of the Manton-Gaulin type.

A portion of the dispersion so prepared is employed for producing apolyethylene terephthalate polymer according to the following procedure:

A polymerization reactor is charged with the following materials:

dimethyl terephthalate monoethylene glycol zinc formate [,000 parts 250parts 0.2 parts (equal to 0.025 mols per l mols of DMTI 0.l6fi parts 400arts The ester interchange and polymerization reactions are carried outwith the same procedure as in example 2 the exception being that theduration of the high vacuum (0.5 mm of mercury) at 280 C., is as long as2 hours 30 mins.

The polymer obtained in the form ofgranules exhibits, uponcrystallization and drying up to a moisture contents of less than 0.01percent, the following specifications:

[n] 0.645 (measured as in example 1) Melt index 0.070 gr./ 10 mins.

m.p. 263 C.,

Number ofCOOH per l 0 g. of polymer: 33.5

Color white EXAMPLE For comparison purposes a polymerization test iscarried out with a dispersion of elemental antimony in ethylene glycol,prepared as follows: l Synthesis of elemental Sb according to theprocedure reported under (a) in example I 2 Dispersion of 2.4parts of Sbin 240 parts ofethylene glycol.

The dispersion thus obtained exhibits, when observed through amicroscope, a very uneven grit size for the particles or particleaggregates, characterized by average dimensions between 10 and 80microns.

Said dispersion is subjected to a polycondensation test in the reactordescribed in example 1, which is charged with the following materials:

2.4 parts of Sh, equivalent to 0.03% by weight on DMT) The reaction massis subjected to ester interchange and polycondensation reactionsaccording to the same procedure as reported in example i. The formedpolymer is extruded under nitrogen pressure in cold water and convertedinto granules.

Said polymer contains many black lumps, a few of which have a size of afew hundreds of microns and thus they are clearly visible by the unaidedeye.

it has, in addition, the following specifications:

m.p. 263 C.,

Melt index 1.08 gr./l0 mins.

Number ofCOOH per 10 g. 30

What is claimed is:

1. A method for the production of poly-methylene terphalates by reactinga reagent selected from the group consisting of bicarboxylic organicacids and dialkyl esters thereof with ethylene glycol liquid in thepresence 0 a catalyst consisting of a dispersion in the liquid glycol ofantimony having a substantially uniform particle size in the range offrom ID to 20 microns 2. A method for the production of poly-methyleneterphalates according to claim 1 wherein the reaction mass is maintainedunder a vacuum of 0.5 to l mms. of mercury and at a temperature of 220C., to 280 C., during the reaction.

3. A method according to claim 1, wherein powdered antimony is initiallydispersed in ethylene glycol by mechanical stirring, and the dispersionthus obtained is subjected to one or more treatments in a colloid mill,to obtain final dispersion of the antimony particles in the ethyleneglycol in a concentration of from 0.1 percent to 1 percent of antimonyand in the particles size range of from l0 to 20 microns, both thestirring and the colloid mill treatment being carried out in an inertgas environment.

4. A method for the production of poly-methylene terphalates accordingto claim 1, wherein the first-named reagent is dimethyl terephthalate,and is in the proportion of 800 to 1000 parts of dimethyl terephthalateto 250to 490 parts of ethylene glycol.

5. A method for the production of poly-methlene terphalates according toclaim 1, wherein the first-named reagent is terephthalic acid, and is inthe proportion of parts of terephthalic acid to 50 parts of ethyleneglycol, and this mixture, mixed with 0.05 parts of antimony per 25 partsof ethylene glycol, is charged into 200 parts of polymerizedpolyethylene terephthalate.

6. A catalyst for use in the production of poly-methlene terphalates byreacting a compound selected from the group consisting of bicarboxylicorganic acids and dialkyl ethers thereof with ethylene glycol consistingof a dispersion in the ethylene glycol of antimony particles, whosesizes are substantially uniform and are in the range offrom 10 to 20microns.

7. A catalyst as claimed in claim 6, wherein the dispersion has aconcentration of from 0.] percent to 1 percent of antimony.

8. A catalyst as claimed in claim 6, wherein the dispersion of theantimony particles in the ethylene glycol has a concentration of 10percent to 50 percent by weight of antimony in the glycol.

2. A method for the production of poly-methylene terphalates accordingto claim 1 wherein the reaction mass is maintained under a vacuum of 0.5to 1 mms. of mercury and at a temperature of 220* C. to 280* C. duringthe reaction.
 3. A method according to claim 1, wherein powderedantimony is initially dispersed in ethylene glycol by mechanicalstirring, and the dispersion thus obtained is subjected to one or moretreatments in a colloid mill, to obtain final dispersion of the antimonyparticles in the ethylene glycol in a concentration of from 0.1 percentto 1 percent of antimony and in the particles size range of from 10 to20 microns, both the stirring and the colloid mill treatment beingcarried out in an inert gas environment.
 4. A method for the productionof polymethylene terphalates according to claim 1, wherein thefirst-named reagent is dimethyl terephthalate, and is in the proportionof 800 to 1000 parts of dimethyl terephthalate to 250 to 490 parts ofethylene glycol.
 5. A method for the production of polymethyleneterphalates according to claim 1, wherein the first-named reagent isterephthalic acid, and is in the proportion of 125 parts of terephthalicacid to 50 parts of ethylene glycol, and this mixture, mixed with 0.05parts of antimony per 25 parts of ethylene glycol, is charged into 200parts of polymerized polyethylene terephthalate.
 6. A catalyst for usein the production of polymethylene terphalates by reacting a compoundselected from the group consisting of bicarboxylic organic acids anddialkyl ethers thereof with ethylene glycol consisting of a dispersionin the ethylene glycol of antimony particles, whose sizes aresubstantially uniform and are in the range of from 10 to 20 microns. 7.A catalyst as claimed in claim 6, wherein the dispersion has aconcentration of from 0.1 percent to 1 percent of antimony.
 8. Acatalyst as claimed in claim 6, wherein the dispersion of the antimonyparticles in the ethylene glycol has a concentration of 10 percent to 50percent by weight of antimony in the glycol.